最近在学习unix网络编程，现在正在学习tcp的通信。其实，只要建立起了tcp通信，操作远端的计算机就不是什么问题了。正向telnet一样，也是基于tcp/IP协议的。所以这个实验，也算是对telnet功能的一种简单的模拟。
但是，值得注意的问题是关机涉及到系统权限，所以要给运行在Server端的程序以足够的权限，这样才可以在接收到Client端的关机请求时，执行关机。
将会模拟如下执行过程：
1． 执行 mytelnet 跟上参数telnet服务器 IP地址 127.0.0.1
2． 输入login 向服务器请求登录，随之服务器会要求输入密码
3． 输入一个错误的登录密码 123
4． 服务器验证不通过，所以返回Fail to login ,please check your password
5． 再次输入密码，这次输入正确的密码123456
6． 服务器验证通过，此时客户端可以执行基于telnet的远程操控
7． 输入操控命令，例如关机命令shutdown
8． 服务器将会执行shutdown 操作。
在编译时使用如下的命令：
Server:
sudo gcc server.c -o server //足够的权限
sudo chmod u+s server
./server
Client:
gcc client.c -o mytelnet
./mytelnet 10.3.1.210
Client端：
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#define MAXLINE 4096
void clientWork(FILE* fp , int sockfd)
{
char sendline[MAXLINE],receiveline[MAXLINE];
int n;
while(fgets(sendline,MAXLINE,fp)!=NULL)
{
write(sockfd,sendline,strlen(sendline));
n=read(sockfd,receiveline,MAXLINE);
receiveline[n]='\0';
fputs(receiveline,stdout);
if(strcmp(receiveline,"bye\n")==0) break;
}
}
int main( int argc , char * * argv )
{
int sockfd , n ;
char recvline[ MAXLINE + 1];
struct sockaddr_in servaddr;
if( ( sockfd = socket( AF_INET , SOCK_STREAM , 0 ) ) < 0 ) {
printf( "socket error\n" );
exit( 1 );
}
memset( &servaddr , 0 , sizeof( servaddr ) );
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons( 2000 );//指定Server端的端口号
char * serverAddress=argv[1];
//判断指定的ip地址是否有错误
if( inet_pton( AF_INET ,serverAddress , &servaddr.sin_addr ) <= 0 ) {
printf( "inet_pton error for %s\n" , serverAddress );
exit( 1 );
}
if( connect( sockfd , (struct sockaddr *)&servaddr , sizeof( servaddr ) ) < 0 ) {
printf( "connect error\n" );
exit( 1 );
}
//从Terminal中读取用户输入的指令
clientWork(stdin,sockfd);
close(sockfd);
return 0;
}
Server端：
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#define MAXSIZE 1024
#define LOGIN 1 //登录Server
#define BYE 2 //离开Server
#define PASS 3 //合法登录
#define DENY 4 //非法登录
#define SHUTDOWN 5 //执行关机
#define ERROR -1 //非法信息
//记录状态信息
static int STATE=0;
int getClientChoice(char * clientmsg)
{
if(strcmp(clientmsg,"login\n")==0) return LOGIN;
if(STATE>0)
{
if(strcmp(clientmsg,"bye\n")==0) return BYE;
}
if(STATE==LOGIN)
{
//Default password is 123456
if(strcmp(clientmsg,"123456\n")==0) return PASS;
else return DENY;
}
if(STATE==PASS)
{
if(strcmp(clientmsg,"shutdown\n")==0) return SHUTDOWN;
else ERROR;
}
return ERROR;
};
//发送消息
void sendMsg(int sockfd,char * buffer,char * msg)
{
char buffer2[MAXSIZE];
snprintf(buffer2,sizeof(buffer2),"%s",msg);
write(sockfd,buffer2,sizeof(buffer2));
}
//定义关机函数
void myshutdown()
{
//系统在一分钟后关机
system("shutdown -t 1");
}
int main(int argc , char * * argv )
{
char buffer[MAXSIZE];
int listenfd=socket(AF_INET,SOCK_STREAM,0);//定义socket，指向ipv4地址的字节流套接口
struct sockaddr_in serverAddr;
memset(&serverAddr,0,sizeof(serverAddr));//sockAddr_in 进行初始化
serverAddr.sin_family=AF_INET;
serverAddr.sin_addr.s_addr=htonl(INADDR_ANY);
serverAddr.sin_port=htons(2000);
if(bind(listenfd,(struct sockaddr *) &serverAddr,sizeof(serverAddr))==-1)
{
printf("There is an error during binding\n");
return -1;
}
else
{
printf("Bind successfully!!!\n");
}
//对listenfd进行监听，从最初建立时的主动套接口（用于进行connect的套接口）转化为被动套接口（接受连接）
listen(listenfd,100);//第二个参数为套接口排队的最大连接个数
int connectfd;
socklen_t addrlen;
struct sockaddr_in connectAddr;
memset(&connectAddr,0,sizeof(connectAddr));
printf("Be ready to accept a connection!\n");
while(1)
{
connectfd=accept(listenfd,(struct sockAddr * )&connectAddr,&addrlen);//接受client端一个请求的socket
char receivebuffer[MAXSIZE];
int revbuflen;
while(1)
{
revbuflen = read(connectfd,receivebuffer,MAXSIZE);
receivebuffer[revbuflen]='\0';
//printf("%s",receivebuffer);
int clientChoice=getClientChoice(receivebuffer);
// 模拟操作过程
if(clientChoice==LOGIN)
{
STATE=LOGIN;
sendMsg(connectfd,buffer,"Please input your password :\n");
}
else if(clientChoice==PASS&&STATE==LOGIN)
{
STATE=PASS;
sendMsg(connectfd,buffer,"Welcome to my Telnet Server...\n");
}
else if(clientChoice==SHUTDOWN&&STATE==PASS)
{
myshutdown();
sendMsg(connectfd,buffer,"Remote computer is going to shutdown...\n");
}
else if (clientChoice==BYE)
{
sendMsg(connectfd,buffer,"bye\n");
break;
}
else if(clientChoice==DENY)
{
sendMsg(connectfd,buffer,"Fail to login ,please check your password\n");
}
else if(clientChoice==ERROR)
{
sendMsg(connectfd,buffer,"wrong,Check your input...\n");
}
}
close(connectfd);
}
close(listenfd); //虽然因为上面有while（true），这行永远都执行不了，但是时刻注意关闭socket连接应该是个好习惯。
return 0;
}